The United States Northern Command, the expansive military command established after 9/11 to protect all American territory ­— in the air, on land, and at sea — is now tracking hurricanes, floods, droughts, wildfires, even major public gatherings.

In regular exercises, it is preparing to respond to nearly every conceivable event where federal troops might be needed to assist civilian authorities ­— from a terrorist attack to an earthquake or a major oil spill.

“I think there is an expectation that we will be able to defend the country against a rogue [missile] threat. I think there is an expectation that we will be there in another [Hurricane] Katrina,” said Army General Charles H. Jacoby, who runs both Northern Command and the North American Aerospace Defense Command, the joint US-Canadian organization that dates to the 1950s and is better known as NORAD.

Yet the command is being asked to do more than anticipated, raising anew questions about the proper role of the active-duty military in domestic affairs, which has been strictly controlled by the Constitution and laws dating to the 19th century.

As a result, it now has the largest legal office of any combatant command, with 17 full-time lawyers. It is the only such office headed by a Coast Guard officer, in a nod to domestic sensitivities.

The legal team is responsible for ensuring that the only active-duty military command responsible for operating on American soil doesn’t violate the law by taking on missions that should be fulfilled by state-controlled National Guard troops or civilian agencies — such as restoring order in disaster areas or operating surveillance aircraft over US territory.

“I don’t think they could have envisioned all of the possible things [the Department of Defense] could have gotten involved in,” said Coast Guard Captain Tim Connors, a native of Sudbury who runs the legal team as chief judge advocate. “But we’re doing a whole lot now that requires constant legal attention.”

Connors expects the issues to get more complicated as budget cuts reduce the ability of state and federal law enforcement and relief agencies to meet demands and the military is asked to fill the void.

“You are going to have to go back to the constitutional framework,’’ he said, “and have those debates.’’

About 1,600 military and civilian personnel are permanently assigned to Northern Command, 50 percent more than when it was established a decade ago. Its annual budget of $168 million, while still small in Pentagon terms, has more than doubled.

Northcom was controversial when it opened its doors in October 2002, taking over the NORAD mission that had been run out of Cheyenne Mountain since the height of the Cold War.

“There were reasons we had not done it before,” said Steven Bucci, deputy assistant secretary of defense for homeland defense in the George W. Bush administration. “They didn’t want the Department of Defense stomping around the United States.”

“I think there is an expectation that we will be there in another [Hurricane] Katrina.” — General Charles Jacoby

At the time, the American Civil Liberties Union called Northcom’s creation a dramatic departure from tradition and raised concerns about blurring the lines between civilian and military that have not abated since.

“The activities are being conducted in such secrecy that we have little ability to monitor,” said Michael German, the ACLU’s senior policy counsel.

He expressed particular concern over the types of potential domestic intelligence involved in the command’s activities. “We don’t have a lot of transparency of how it is being utilized,” he said.

Watchdog groups expressed concern when Northcom, which has few dedicated troops and draws on additional forces as needed, assigned active-duty units to be permanently on call to respond to the potential use of chemical, biological, or nuclear weapons. According to the command, that mission now has more than 6,000 active-duty troops assigned to it.

One of the primary legal guidelines it must hew to is the Posse Comitatus Act of 1878, which prohibits the direct use of the armed forces in law enforcement, except in extreme cases when ordered by the president or in special situations expressly approved by Congress.

The command’s domestic support role has not been without growing pains. Bucci, who has participated in homeland defense exercises, said at times senior generals have had difficulty getting used to the idea of answering to civilian authorities.

In one case, a simulation of a large-scale terrorist attack in Indianapolis, they wanted to take control of the city. The request was denied, and in a subsequent session the military representatives changed their position.

“In between, someone had explained the Constitution to them,” Bucci recalled of the 2007 exercise.

The command’s varied missions — and growing support role — were on display earlier this month in its highly secure, 24-hour command center, where maps of the United States, Canada, and Mexico hang on the walls and flicker on computer monitors and scenes of Mount Rushmore and the Rio Grande fill the hallways.

There are consoles dedicated to monitoring traditional threats such as missile launches — the only clock on the wall showing time overseas depicts North Korea — and others where technicians monitor 60,000 civilian airline flights a day with Federal Aviation Administration feeds.

Nearby are consoles dedicated to a broader array of missions not traditionally under the purview of the armed forces.

One is monitoring feeds of hundreds of ships entering US waters and plying inland waterways; another keeps tabs on natural and man-made disasters and large public events considered possible terrorist targets such as political conventions, the Super Bowl, and recently the UN General Assembly meeting in New York.

Last year, Northcom conducted a major exercise to simulate the aftermath of a rupture in the New Madrid fault in Missouri, the type of “low-likelihood, high-consequence” event for which Jacoby, the Northcom commander, says the military is uniquely qualified to provide humanitarian assistance.

“It’s a diverse mission set,” he said.

But he stressed the command only operates in support of civilian authorities — and only when the military has assets or expertise that the National Guard or civilian agencies like the Federal Emergency Management Agency do not.

“Our center of gravity, the way we do our business, is through our partnerships, our relationships with other agencies,” Jacoby said.

Although Northern Command operates out of the massive bunker built into Cheyenne Mountain, its headquarters are in nearby Peterson Air Force Base in Colorado Springs. About 60 federal and state agencies are also represented in the headquarters.

“We have built some amazing, trusted relationships that are really key to how the Department of Defense can support in the homeland,” Jacoby said.

Still, it is a role that took some getting used to for their civilian counterparts as well.

“They were intimidated,” recalled retired Army Lieutenant General Edward G. Anderson III, who served as Northcom’s first deputy commander between 2002 and 2004.

Their response, Anderson said, was often: “You’re the Department of Defense. You’re coming in here taking over everything.

“We told them we only come if the president or governor asks us to,” Anderson said. “We are there to support. Not to take over.”

Yet such concerns explain why the legal staff is often engaged in lively debate.

Often it is over a request for federal troops to check on neighborhoods struck by floods or hurricanes, according to Connors, the Coast Guard lawyer.

“That can be problematic if the situation is chaotic,” he said. “You are traipsing close to that line. Situations can become tenuous from a law enforcement perspective [with] lots of lawless behavior.

“You don’t want to violate someone’s privacy,” he added. “You want to use that [Pentagon aircraft] to look at places that are damaged. You don’t want to look into people’s houses.”

Command officials say they adhere to a strict set of rules governing intelligence.

“We don’t do domestic intelligence,” said Army Major General Francis Mahon, the command’s chief of planning

Connors went a step further, saying that while the command can be privy to some intelligence — say, on a potential terrorist threat — “We are not going to collect it, and we are not going to hold it.”

But as the command’s activities continue to expand — and the potential threats to American citizens evolve — the legal thicket is bound to require more attention.

Asks Jacoby: “Where is the next growth?”

One threat he said is increasingly on the minds of officials here is the potential for retaliatory attacks on American soil if the nation gets involved in another conflict overseas, such as in Iran or Syria.

“We’ve got work to do but we are actively thinking through that now,” he said. “Our job is to do the ‘what if’ drill.”

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